1. Field of the Invention
The present invention relates to a dimming control circuit and, more particularly, to a dimming control circuit applied to a drive circuit for driving light-emitting diodes.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a conventional light-emitting diode drive circuit 10. In the example of FIG. 1, the light-emitting diode drive circuit 10 is implemented by a boost-type switching voltage regulator for converting an input voltage Vin into an output voltage Vout desired for driving one or more series-connected light-emitting diodes LED. In accordance with a current IL flowing through an inductor L and a feedback voltage Vfb from a resistor R, a switching control circuit 11 generates a fixed-duty pulse drive signal FS for turning on/off a switching transistor Q. The duty ratio of the switching transistor Q determines the proportional relationship between the output voltage Vout and the input voltage Vin. The brightness of the light-emitting diodes LED varies depending on the diode current ILED flowing through themselves. From FIG. 1 is derived an equation regarding to the diode current ILED: ILED=Vfb/R=(Vout−N*Vd)/R, where N is the number of the light-emitting diodes and Vd is a voltage drop of one single conductive light-emitting diode. Since the voltage drop Vd may be considered approximately constant, the diode current ILED as well as the brightness of the light-emitting diodes LED is easily controlled by the adjustment to the output voltage Vout.
Another method of controlling the brightness of the light-emitting diodes LED appeals to the nature of human-eye perceptions. For bright-dark cycles alternating over about 60 Hz, the human eyes perceive an average brightness instead of flickering. In the bright phase the switching transistor Q is, as conventional, turned on/off by the fixed-duty pulse drive signal FS from the switching control circuit 11, but in the dark phase the fixed-duty pulse drive signal FS is blocked in order to keep the switching transistor Q nonconductive. In other words, through controlling the ratio of the bright phase to the dark phase, the desired average brightness is achieved. However, such a dimming method by using bright-dark cycles causes a huge current noise peak at the beginning of each bright phase. Because the frequency of the bright-dark cycles may be set within the audio-frequency range, the serially-occurred current noise peaks actually produce noisy sounds to human ears.